Fullerene Blends Research Articles

Real-Time Tracking of Polymer Crystallization Dynamics in Organic Bulk Heterojunctions by Raman Microscopy

State-of-the-art organic photovoltaic active layers typically undergo post-treatment such as thermal or solvent vapor annealing to increase their performance by tuning the bulk heterojunction morphology. Molecular crystallinity is one of the key factors that determine the morphology. Real-time tracking of the crystallinity during the post-treatment is strongly desired for understanding the physics of the crystallization process and for optimizing the post-treatment protocol. Here, we report on the cold crystallization (CC) dynamics of the polymer in the temperature range of 50–150 °C in polymer:fullerene blends based on poly(3-hexylthiophene) with various fullerene-based acceptors (C60, PC61BM, PC71BM, bisPC61BM, HBIM, AIM8, and IrC60) in real-time by Raman microscopy. We also reveal how different solvents, fullerene acceptors, and temperatures affect CC during thermal annealing. We further demonstrate a correlation between the fullerene derivative weight and the polymer crystallinity for the as-cast film...

Degradation kinetics in different polymer–fullerene blends investigated by electron spin resonance

Abstract

Increased Exciton Delocalization of Polymer upon Blending with Fullerene.

Interfaces between donor and acceptor in a polymer solar cell play a crucial role in exciton dissociation and charge photogeneration. While the importance of charge transfer (CT) excitons for free carrier generation is intensively studied, the effect of blending on the nature of the polymer excitons in relation to the blend nanomorphology remains largely unexplored. In this work, electroabsorption (EA) spectroscopy is used to study the excited-state polarizability of polymer excitons in several polymer:fullerene blend systems, and it is found that excited-state polarizability of polymer excitons in the blends is a strong function of blend nanomorphology. The increase in excited-state polarizability with decreased domain size indicates that intermixing of states at the interface between the donor polymers and fullerene increases the exciton delocalization, resulting in an increase in exciton dissociation efficiency. This conclusion is further supported by transient absorption spectroscopy and time-resolved photoluminescence measurements, along with the results from time-dependent density functional theory calculations. These findings indicate that polymer excited-state polarizability is a key parameter for efficient free carrier generation and should be considered in the design and development of high-performance polymer solar cells.

Optical and Morphological Properties of P3HT and P3HT: PCBM Thin Films Used in Photovoltaic Applications

This work is focused on the study of some physical properties of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(e-hexylthiophene-2,5-diyl): Methanolfullerene Phenyl-C61-Butyric-Accid-Methyl-Ester (PCBM) blend thin films. Knowing the polymer advantages, such as ease of processing, high thermal stability, strong interaction with light, its properties have captured the attention regarding the changes that can occur in a polymer:fullerene blend in term of them. Polymer and polymer:fullerene blend (1:0.1, 1:0.2, 1:0.4 and 1:0.8 ratios) were deposited by spin coating on glass and SnO2:F (FTO) coated glass. The optical properties were emphasized using spectrophotometry (300 – 2200 nm wavelength range) and spectroscopic ellipsometry models, to obtain the refractive index, extinction coefficient and the transmission (found higher than 80%). According to X-ray diffraction analysis, as-obtained films are amorphous. Investigation of the surface morphology of thin-film samples using Atomic Force Microscopy revealed a crystallite-like surface morphology with crystallite size in the nanometer range.

Environmentally friendly preparation of nanoparticles for organic photovoltaics

Aqueous nanoparticle dispersions were prepared from a conjugated polymer poly[thiophene-2,5-diyl-alt-5,10-bis((2-hexyldecyl)oxy)dithieno[3,2-c:3′,2′-h][1,5]naphthyridine-2,7-diyl] (PTNT) and fullerene blend utilizing chloroform as well as a non-chlorinated and environmentally benign solvent, o-xylene, as the miniemulsion dispersed phase solvent. The nanoparticles (NPs) in the solid-state film were found to coalesce and offered a smooth surface topography upon thermal annealing. Organic photovoltaics (OPVs) with photoactive layer processed from the nanoparticle dispersions prepared using chloroform as the miniemulsion dispersed phase solvent were found to have a power conversion efficiency (PCE) of 1.04%, which increased to 1.65% for devices utilizing NPs prepared from o-xylene. Physical, thermal and optical properties of NPs prepared using both chloroform and o-xylene were systematically studied using dynamic mechanical thermal analysis (DMTA) and photoluminescence (PL) spectroscopy and correlated to their photovoltaic properties. The PL results indicate different morphology of NPs in the solid state were achieved by varying miniemulsion dispersed phase solvent.

Probing the pathways of free charge generation in organic bulk heterojunction solar cells

The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

Morphology and electrical characteristics of polymer: Fullerene films deposited by electrospray

Electrospray (ES) has emerged as an attractive approach to fabricate thin-film organic photovoltaic devices. However, the morphological properties and their implication to electrical characteristics of polymer:fullerene blends deposited by this method have not been clearly understood yet. In order to uncover the interplay of nanoscale morphology and electrical properties of ES-deposited films, both the residues of single droplets and thin films from the overlapping of multiple droplets were investigated by various characterization techniques. The surface morphology was demonstrated in details by the optical microscope (OM) and atom force microscope (AFM). The ordering structures and aggregation of the polymer were identified by the Raman and UV–vis spectroscopic investigations. The spatially distribution map of the P3HT aggregation states was analyzed by fitting the C˭C mode of P3HT with Lorentzian functions. The conductive atomic force microscopy (C-AFM) was used to quantify local currents and reveal the correlation between the nanostructure and charge-transport mobility. Both surface morphology and aggregation of the P3HT were found to strongly depend on the overlapping boundaries formed by the dry residues of individual droplets. More P3HT aggregation present at the boundaries. However, boundaries also show significantly higher charge-transport resistance thus lower current. Thin films deposited with less droplet evaporation exhibit more homogenous morphology, more uniform phase segregation, and consequently higher charge mobility.

Organic solar cells based on anthracene-containing PPE–PPVs and non-fullerene acceptors

Lately, non-fullerene acceptors (NFAs) have received increasing attention for use in polymer-based bulk-heterojunction (BHJ) organic solar cells (OSCs), as improved photovoltaic performance compared to classical polymer–fullerene blends could be demonstrated. In this study, polymer solar cells based on a statistically substituted anthracene-containing poly(p-phenylene ethynylene)-alt-poly(p-phenylene vinylene)s (PPE–PPVs) copolymer (AnE-PVstat) as donor in combination with a number of different electron accepting materials were investigated. Strong photoluminescence quenching of the polymer donor indicates intimate intermixing of both materials. However, the photovoltaic performances were found to be poor compared to blends that use fullerene as acceptor. Time-delayed collection field (TDCF) measurements demonstrate: charge generation is field-independent, but bimolecular recombination processes limit the fill factor and thus the efficiency of devices.

Morphological, Chemical, and Electronic Changes of the Conjugated Polymer PTB7 with Thermal Annealing

SummaryThere is considerable interest in improving the performance of organic optoelectronic devices through processing techniques. Here, we study the effect of high-temperature annealing on the properties of the semiconducting polymer PTB7 and PTB7:fullerene blends, of interest as efficient organic photovoltaic (OPV) devices. Annealing to moderate temperature improves the PTB7 morphology and optoelectronic properties. High-temperature annealing also improves morphology but results in poorer optoelectronic properties. This is a result of side chain cleavage that creates by-products that act as trap states, increasing electronic disorder and decreasing mobility. We further observe changes to the PTB7 chemical structure after thermal cleavage that are similar to those following solar irradiation. This implies that side chain cleavage is an important mechanism in device photodegradation, which is a major “burn-in” loss mechanism in OPV. These results lend insight into side chain cleavage as a method of improving optoelectronic properties and suggest strategies for improvement in device photostability.

Low temperature processed high-performance thick film ternary polymer solar cell with enhanced stability

Recently, efficient thick-film polymer solar cells (PSCs) were reported to be promising candidates for large-area module device fabrication because of their potential compatibility with roll to roll processing technique. Nevertheless, most of thick-film PSCs with high power conversion efficiencies (PCEs) were fabricated under high temperature (over 100 °C, namely “Hot-Processing”), which restricts their potential commercialization application. Herein, a new pathway was developed to make high performance thick-film PSCs under a mild condition without hot-processing. It was found that the ternary strategy can efficiently tune the processing conditions of thick film PSCs due to the decrease of polymer aggregation intensity. Consequently, high-efficiency thick-film ternary PSCs (10.59%) without hot-processing are designed and fabricated by introducing the nematic liquid-crystalline small molecule (benzodithiophene terthiophene rhodanine, BTR) into fluorinated benzothiadiazole-based polymer (PffBT4T-2OD):fullerene host blend. The morphology studies reveal that the incorporation of BTR has a similar effect as hot-processing on the blend films, resulting in a much improved nano-scale phase separation. It was also demonstrated that the addition of BTR is beneficial for improving light harvesting, charge separation, transport and extraction of the resulting devices. Moreover, this ternary strategy shows a well general applicability for other systems that need hot-processing. Such as for naphthobisthiadiazole-based polymer PNTT, the optimized ternary device without hot-processing obtained an improved performance with a PCE of 11.44%, which is one of the highest-efficiency thick-film PSCs reported to date. Therefore, our results provide a facile approach to fabricate the high performance PSCs with thick-active layer at a lower temperature instead of hot-processing and may meet the needs of future roll-to-roll production of PSCs.

Effect of Replacing Alkyl Side Chains with Triethylene Glycols on Photovoltaic Properties of Easily Accessible Fluorene-Based Non-Fullerene Molecular Acceptors: Improve or Deteriorate?

Hydrophilic oligo(ethylene glycol) (OEG) side chains are more flexible than alkyl chains and can facilitate the π–π stacking of conjugated polymer backbones, which has been proven to be a key structural feature leading to better device performance in polymer:fullerene blend polymer solar cells (PSCs). So far, little has been known about the influence of OEG side chains on the performance of non-fullerene acceptors in polymer:non-fullerene acceptor blends. Based on an easily accessible conjugated backbone of dicyanomethylene indanone–thiophene–fluorene–thiophene–dicyanometnylene indanone (DICTF), two non-fullerene molecular acceptors were synthesized with decyl (DICTF-C10) and triethylene glycol monoether (DICTF-TEG) side chains to blend with the polymer donor of PTB7-Th for PSCs. Replacing the decyl side chains with the TEGs on DICTF does improve the π–π stacking between the conjugated backbones as expected. For DICTF-TEG, nevertheless, forming the BHJ blend with the PTB7-Th donor leads to unfavorable fil...

Simulating Phase Separation during Spin Coating of a Polymer–Fullerene Blend: A Joint Computational and Experimental Investigation

During spin coating of the photoactive layer of a bulk heterojunction organic solar cell, phase separation between the donor (D) and acceptor (A) components is triggered by solvent evaporation. The morphology of the resulting layer is one of the main determinants of the device efficiency and critically depends on processing conditions such as the spinning speed, D–A mixing ratio, and choice of solvents. It is crucial to understand how these conditions influence the nanostructure of the photoactive layer. Optical experiments have a limited spatial resolution and cannot probe the short length scales of phase separation. In this work, we present three-dimensional simulations of evaporation-induced phase separation in a diketopyrrolopyrrole–fullerene D–A blend, where we derive the simulation parameters from in situ laser interference and contact angle experiments. Depending on the drying rate, phase separation initiates in different regions of the thinning film. From a linear stability analysis, we estimate t...

Polymer Solar Cells with 90% External Quantum Efficiency Featuring an Ideal Light- and Charge-Manipulation Layer.

Rapid progress in the power conversion efficiency (PCE) of polymer solar cells (PSEs) is beneficial from the factors that match the irradiated solar spectrum, maximize incident light absorption, and reduce photogenerated charge recombination. To optimize the device efficiency, a nanopatterned ZnO:Al2 O3 composite film is presented as an efficient light- and charge-manipulation layer (LCML). The Al2 O3 shells on the ZnO nanoparticles offer the passivation effect that allows optimal electron collection by suppressing charge-recombination loss. Both the increased refractive index and the patterned deterministic aperiodic nanostructure in the ZnO:Al2 O3 LCML cause broadband light harvesting. Highly efficient single-junction PSCs for different binary blends are obtained with a peak external quantum efficiency of up to 90%, showing certified PCEs of 9.69% and 13.03% for a fullerene blend of PTB7:PC71 BM and a nonfullerene blend, FTAZ:IDIC, respectively. Because of the substantial increase in efficiency, this method unlocks the full potential of the ZnO:Al2 O3 LCML toward future photovoltaic applications.

Co-Solvent Effect in the Processing of the Perovskite:Fullerene Blend Films for Electron Transport Layer-Free Solar Cells

An understanding of the improvements achieved in the use of cosolvents for methylammonium lead triiodide (MAPbI3):C70 blend films (MAPbI3:C70) processing is presented. A comparative study using aromatic (i.e., o-xylene and o-dichlorobenzene) and aliphatic (i.e., methylcyclohexane and chlorocyclohexane) cosolvents proves the nature of the cosolvent interacting with fullerene to be determining for achieving enhanced devices. UV–vis spectra of the different C70 solutions suggest a major impact of the solute–aromatic solvent interactions on the optoelectronic properties. The effect of aromatic and aliphatic solvents in the electronic structure of C70 crystals, obtained from the different solutions, is indeed demonstrated by electron energy loss spectroscopy. Morphological studies show elimination of pinholes (field emission scanning electron microscopy) and different nanometric features related to fullerene (atomic force microscopy) in MAPbI3:C70 blend films processed using aromatic cosolvents. A severe quenc...

Optimizing the Crystallinity and Phase Separation of PTB7:PC71BM Films by Modified Graphene Oxide

A facile method is proposed to obtain modified shorn graphene oxide (DDAB-sGO) with improved dispersion in organic solvents. Didodecyl dimethylammonium bromide (DDAB)-sGO, which exhibits good dispersibility in the nonpolar solvent o-dichlorobenzene, was obtained via the sono-Fenton reaction and DDAB ionic functionalization. DDAB-sGO was used in the preparation of conjugated polymer:fullerene blend composites. UV–visible absorption spectra, steady-state photoluminescence spectra, fluorescence decay, and grazing incidence X-ray scattering measurements were applied to characterize morphologies, structural features, and charge-transport characteristics of the composites. Doped into poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) conjugated polymer blends, DDAB-sGO is shown to facilitate increased crystallinity and phase separation of PTB7 and PC71BM to achieve a m...

Terahertz short-range mobilities in neat and intermixed regions of polymer:fullerene blends with controlled phase morphology

Terahertz spectroscopy unravels the inner working principles of polymer:fullerene blend organic semiconductors with regard to their solar cell performance.

Mechanically robust and high-performance ternary solar cells combining the merits of all-polymer and fullerene blends

In this work, we present a facile strategy that combines the merits of both fullerene-PSCs and all-PSCs to significantly increase the PCE of the all-PSCs without compromising mechanical performance.

Engineering interactions in QDs-PCBM blends: a surface chemistry approach.

Here we present a comprehensive study on the photophysics of QDs-fullerene blends, aiming to elucidate the impact of ligands on the extraction of carriers from QDs. We investigated how three different ligands (oleylamine, octadecanethiol and propanethiol) influence the dynamics of charge generation, separation, and recombination in blends of CdSe/CdS core/shell QDs and PCBM. We accessed each relevant process directly by combining the results from both optical and magnetic spectroscopies. Transient absorption measurements revealed a faster interaction dynamics in thiol-capped ligands. Through phenomenological modeling of the interaction processes, i.e., energy transfer and electron transfer, we estimated the suppression of exciton migration and the enhancement of electron transfer processes when alkyl-thiols are employed as ligands. Contextually, we report the profound impact of the ligands' alkyl chain length, leading to strengthened interactions with PCBM acceptors. Quantitatively, we measured a 10-fold increase in the electron transfer rate when oleylamine ligands were exchanged with propanethiol ligands. EPR spectroscopy gave access to subtle details regarding both the enhanced charge generation and lower binding energy of charge-transfer states in blends compared to PCBM alone. Moreover, through pulsed EPR techniques, we inferred the localization of deep electron traps in localized sites close to QDs in the blends. Therefore, our thorough characterization evidenced the essential role of ligands in determining QD interactions. We believe that these discoveries will contribute to the efficient incorporation of QDs in existing organic PV technologies.

Toward Over 15% Power Conversion Efficiency for Organic Solar Cells: Current Status and Perspectives

AbstractSignificant improvement in the power conversion efficiency (PCE) of organic solar cells (OSCs) is achieved by developing novel donor and acceptor materials, optimizing the phase‐separation morphology via diversified strategies, and using interfacial materials for better charge‐carrier collection. For state‐of‐the‐art devices, a PCE of over 13% is reported. However, simulations indicate that an efficiency of ≈19% could be realized, assuming a total energy loss of 0.5 eV with an external quantum efficiency of 90% and a fill factor of 70%. This large difference between the theoretical calculations and the actual performance of the state‐of‐the‐art devices shows that OSCs have significant potential for the future. Here, the energy loss is discussed, which determines the PCE limit, and then different systems are reviewed, such as small‐molecule (SM)/fullerene blends, polymer/fullerene blends, SM/nonfullerene blends, polymer/nonfullerene blends, and multicomponent systems. After highlighting the factors that have limited the device efficiency to date, an outlook on the most important challenges to guide OSCs toward the 15% PCE regime is provided.

A Shockley‐Type Polymer: Fullerene Solar Cell

AbstractCharge extraction rate in solar cells made of blends of electron donating/accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion‐controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high‐efficiency Shockley‐type organic solar cells with junction thicknesses suitable for scaling up.